After reaching the fourth floor and eventually killing everything there, open the box to get your reward. You can also kill the Monk and the Witch, which are easy to kill, grab the reward, and leave. The Dwarf Geomancer is hard to melee on a lower level/on lower skills.

You can then go back down to the bottom of the tower and out, if someone has closed the door on you, you will need to go into the basement and go through the teleporter.